Can I use Space + LMB to pan?
Yes, this is possible.
Open up the File &gt; User Preferences (see 2nd image) by pressing ⎈ Ctrl⎇ AltU and go the the Input section (the third rider on the top menu of the User Preferences window).
First, we have to remove/change the existing bindings for a  LMB as well as Space.

The functionality bound to  LMB click is called Set 3D Cursor. Search for 3D Cursor in the search window. Remove the item by pressing the X - button.

Find the Spacebar command by searching for Space via key-binding. Remove this item as well.

Now search for Rotate View. Click on the items shortcut, then perform a left click again to enter a Left Mouse value. To set the modifier key to space, click on the (currently empty) input box in the right of the third row and press the Space key. It is going to be used as a modifier key for the action.


You can now rotate the view using Space LMB
You have to unbind Set 3D Cursor otherwise the event will not be passed on.

You press Space.
You make a  LMB down click.
Set 3D Cursor is called and all other events are aborted.

You can however set Set 3D Cursor to a  LMB using either Click or Release as the mouse mode. The action will however only be called once you release the mouse.